The present invention relates to disc drive storage systems and, more particularly, to a positive pressure slider having a wide center rail for slow disc speed and high flying height applications, such as optical disc drive storage systems.
In optical disc drive storage systems, data is accessed by focusing a laser beam onto the data surface of a rigid disc and detecting light reflected from or transmitted through the data surface. In general, data is stored in the form of physical or magnetic marks carried on the surface of the disc which are detected using the reflected laser light. There are a number of different optical disc technologies which are known in the industry. For example, CD-ROMs are currently used to store digital data such as computer programs or digitized music. Typically, CD-ROMs are permanently recorded during manufacture. Another type of optical system is write-once read-many (WORM) systems in which a user may permanently write information onto a blank disc. It is also desirable to provide a system which is erasable, such as phase change and magneto-optic (M-O) systems. Phase change systems detect data by sensing a change in reflectivity. M-O systems detect data by measuring the rotation of the incident light polarization due to the magnetic orientation of the storage medium.
High density optical recording, particularly near-field recording (i.e., M-O or phase change systems), typically requires an optical head gimbal assembly (OHGA) having a slider for carrying an optical element over the data surface of the optical media. U.S. Pat. No. 5,497,359, issued Mar. 5, 1996, entitled "OPTICAL DISC DATA STORAGE SYSTEM WITH RADIATION-TRANSPARENT AIR-BEARING SLIDER" shows an example of a slider for use with an optical disc drive storage system.
In order to write a magnetic bit of information onto the disc surface, the disc surface is optically heated. With M-O media, for example, the laser beam is directed through an optical aperture in the slider, which heats the disc surface to a point above the Curie temperature of the medium. A magnetic coil carried on the slider is energized and the laser is turned off. As the medium cools below the Curie temperature, the heated spot is left with a desired magnetic orientation.
An actuator mechanism moves the slider from track to track across the surface of the disc under the control of electronic circuitry. The actuator mechanism includes a track accessing arm and a suspension for each optical head gimbal assembly. The suspension includes a load beam and a gimbal. The load beam provides a preload force which forces the slider toward the disc surface. The gimbal is positioned between the slider and the load beam to provide a resilient connection that allows the slider to pitch and roll while following the topography of the disc. The slider has an air bearing surface which faces the disc surface.
There are generally two types of sliders used in the disc drive industry, positive pressure air bearing (PPAB) sliders and self-loading or "negative pressure" air bearing (NPAB) sliders. An NPAB slider typically has a pair of rails extending along the sides of the bearing, with a cavity dam or cross bar extending between the rails near the leading end of the slider. As the disc rotates, the surface of the disc drags air under the cavity dam by viscous friction. As the air passes over the cavity dam, the air expands into a "cavity" between the rails, which forms a partial vacuum in the cavity. The partial vacuum draws the slider closer to the disc surface and counteracts positive pressure developed along the rails. The cavity is open to atmospheric pressure at the trailing end of the slider, and may also include a center rail or an island at the trailing end of the slider to mount a single recording head. NPAB surfaces have many advantages, such as reduced take off and landing velocity during spindle start up and shut down, high bearing stiffness and lower sensitivity of flying height to changes in altitude and velocity, as compared to PPAB sliders having no cavity dam.
However, NPAB sliders are seldom used in ramp "load-unload" drive applications because of their high suction force. In these applications, the slider is unloaded from the disc surface by rotating the actuator mechanism until the suspension engages a ramp which lifts the suspension and thus the slider from the disc surface. The high suction force prevents an NPAB slider from following the suspension as the suspension rides up on the unloading ramp. The slider remains in close proximity to the spinning disc, and the ramp elastically deforms the suspension. The NPAB suction force breaks only when a significant elastic strain has accumulated in the suspension. The release of the suction force releases the elastic strain in the suspension and allows the slider to unload from the disc surface. This cycle of suction force and strain release occurs very rapidly relative to the time in which the suspension is in contact with the unloading ramp. The rapid release of elastic strain energy sets up vibratory oscillations in the slider position coordinate that is normal to the plane of the disc surface. These oscillations may be large enough to cause the slider to "slap" against the disc surface, thereby generating wear debris particles and possibly damaging the recording head.
Another problem observed with NPAB sliders during unloading occurs in the event that the unloading force exerted by the deformed suspension is too small to overcome the suction force. In this event, the suction force is broken when the slider is swung over the disc perimeter, allowing the atmosphere to flow into the cavity between the side rails with very little resistance. As the slider passes over the disc perimeter, pressurization between the side rails becomes unbalanced, causing the slider to roll to one side. As a result, the slider may contact the disc perimeter when unloading. Repetition of such contact causes wear on the slider and generates debris particles.
In contrast, PPAB sliders have a low suction force, making them more applicable for ramp load-unload drive applications than NPAB sliders. Although the side rails in a PPAB slider are not connected by a cavity dam, some air expansion typically occurs as the air is dragged under slider, if the slider has side rails with wide leading ends that transition to a narrow sections near the middle of the slider. The suction force due to these expansions is somewhat smaller than that obtained with the expansion over a cavity dam in an NPAB slider, which allows PPAB sliders to be used more effectively in ramp load-unload applications. A commonly encountered problem in PPAB sliders is that the flying height is much higher at the disc outer diameter (OD) than at the disc inner diameter (ID), particularly for applications having a low disc speed and a relatively high flying height, such as in optical disc drive storage systems. This can cause a reduction in the overall recording density achieved by the disc drive.